A steering wheel is attached to an upper end of a steering shaft in order to steer an automobile. When a driver rotates the steering wheel, a steering mechanism is operated and the direction of steered wheels is changed, whereby steering action is provided.
For example, referring to the appended drawings as illustrated in FIG. 6(A), as a conventional steering-wheel core, a core 200 in which a spoke 221 extending outward is attached to a boss 231 provided at the center and a circular rim 211 is coupled to an outer circumference of the spoke 221 is known. A soft synthetic resin coating such as, for example, a foamed polyurethane resin is formed on such a core 200, whereby a steering wheel is manufactured (for example, see Japanese Patent Application Publication No. 2000-301998).
Since the steering wheel is rotated while being held by the operator's hands as described above and the rotation direction thereof is changed frequently, the steering wheel needs to have rigidity. Therefore, metals such as iron, aluminum, or magnesium are often used for the structure of the steering wheel.
In recent years, a method of integrally molding a steering-wheel core by casting using a magnesium alloy having low density as a material has been widely used in vehicles including automobiles.
Present methods of casting using a magnesium alloy as a material has the following problem. When a high-temperature and high-pressure molten magnesium alloy is caused to flow into a die, if the molten magnesium alloy makes direct contact with an inner surface of the die, the molten magnesium alloy flows continuously into the contacting portion. Therefore, a local hot spot is formed and the surface of a product after casting may become rougher than a specific surface roughness.
Particularly, when a steering wheel and a steering shaft are coupled, although a nut N (see FIG. 6(B)) is fastened to a distal end side (S3 in FIG. 6(B)) of the steering shaft, a nut seat surface 235 where the nut N and a boss 231 make contact with each other needs to have a clean casting surface in order to control the torque of the nut N. However, in the conventional technique, the surface roughness of the nut seat surface may exceed a specified roughness after casting. In this case, it is necessary to perform finishing processing such as grinding on the surface of a product after casting, which imposes manufacturing cost penalties.
In view of such a problem, an object of the present invention is to provide a steering-wheel core and a method for casting the same capable of eliminating the need to perform finishing processing after casting and decreasing a manufacturing cost, even when the steering-wheel core is cast using a molten metal.